This application is a 371 of PCT/EP01/07427 filed Jun. 29, 2001.
The present invention relates to a process for preparing polytetrahydrofuran or tetrahydrofuran copolymers by polymerization of tetrahydrofuran over a heterogeneous, acid catalyst in the presence of at least one telogen and/or comonomer selected from the group consisting of alpha, omega-diols, water, polytetrahydrofuran having a mean molecular weight of from 200 to 700 dalton and cyclic ethers with at least part of the unreacted tetrahydrofuran being returned to the polymerization.
Polytetrahydrofuran, hereinafter referred to as PTHF, which is also known as polyoxybutylene glycol is used as a versatile intermediate in the plastics and synthetic fibers industry and is employed, inter alia, for preparing polyurethane, polyester and polyamide elastomers. Furthermore, it, like some of its derivatives, is a valuable auxiliary in many areas, for example as dispersant or in the deinking of waste paper.
PTHF is usually prepared industrially by polymerization of tetrahydrofuran, hereinafter referred to as THF for short, over suitable catalysts. Addition of suitable reagents makes it possible to control the chain length of the polymer chains and thus to set the mean molecular weight to the desired value. Such reagents are termed chain termination reagents or xe2x80x9ctelogensxe2x80x9d. This control is achieved by choice of type and amount of the telogen. Additional functional groups can be introduced at one or both ends of the polymer chain by choice of suitable telogens.
Thus, for example, use of carboxylic acids or carboxylic anhydrides as telogens makes it possible to produce the monoesters or diesters of PTHF, which subsequently have to be converted in to PTHF by saponification or transesterification. These processes are therefore known as two-stage PTHF processes.
Other telogens act not only as chain termination reagents but are also incorporated into the growing polymer chain of the PTHF. They not only have the function of a telogen, but at the same time act as a comonomer and can therefore be equally well designated as telogens and as comonomers. Examples of such comonomers are telogens having two hydroxy groups, e.g. diols (dialcohols). These can be, for example, ethylene glycol, propylene glycol, butylene glycol, 1,3-propanediol, 1,4-butanediol, 2-butyne-1,4-diol, 1,6-hexanediol or low molecular weight PTHF. Further suitable comonomers are cyclic ethers such as 1,2-alkylene oxides, for example ethylene oxide or propylene oxide, 2-methyltetrahydrofuran or 3-methyltetrahydrofuran. With the exception of water, 1,4-butanediol and low molecular weight PTHF, the use of such comonomers leads to the preparation of tetrahydrofuran copolymers, hereinafter referred to as THF copolymers, and in this way makes it possible to modify PTHF chemically.
In industry, use is made predominantly of two-stage processes in which tetrahydrofuran is, for example, polymerized in the presence of fluorosulfonic acid to form polytetrahydrofuran esters which are subsequently hydrolyzed to polytetrahydrofuran. Alternatively, tetrahydrofuran is, for example, polymerized with acetic anhydride in the presence of acid catalysts to form polytetrahydrofuran diacetate which is subsequently transesterified, for example with methanol, to give polytetrahydrofuran. Disadvantages of such processes are that they have to be carried out in two stages and that by-products such as hydrofluoric acid and methyl acetate are formed.
The single-stage synthesis of PTHF is carried out by polymerization of THF using water, 1,4-butanediol or low molecular weight PTHF as telogen over acid catalysts. Known catalysts are both systems homogeneously dissolved in the reaction system and also heterogeneous, i.e. largely undissolved, systems.
EP-B-126 471 describes water-containing heteropolyacids, for example tungstophosphoric acid, for the single-stage synthesis of PTHF using water as telogen, and EP-B-158 229 discloses the same catalyts for the single-stage synthesis of PTHF using diols such as 1,4-butanediol as telogen. Under the reaction conditions described, two liquid phases, namely one catalyst-containing phase in which not only THF but also the major part of the heteropolyacid and the water are present and an organic phase comprising mainly THF, PTHF and residual amounts of catalyst, are formed. Since it is difficult to separate off the homogeneous catalyst, the heterogeneously catalyzed processes have gained importance for the single-stage synthesis of PTHF and also the direct synthesis of THF copolymers.
According to U.S. Pat. No. 4,120,903, PTHF can be prepared from THF and water with the aid of superacid Nafion(copyright) ion exchange resins.
DE-A 44 33 606 describes, inter alia, a process for preparing PTHF by polymerization of tetrahydrofuran over a heterogeneous catalyst in the presence of one of the telogens water, 1,4-butanediol, PTHF having a molecular weight of from 200 to 700 dalton or mixtures of these telogens, where the catalyst is a supported catalyst which comprises a catalytically active amount of an oxygen-containing tungsten or molybdenum compound or mixtures of these compounds on an oxidic support material and has been calcined at from 500xc2x0 C. to 1000xc2x0 C. after application of the precursor compounds of the oxygen-containing molybdenum and/or tungsten compounds. DE-A 196 49 803 discloses the use of promoters to increase the activity of the catalysts described in DE-A 44 33 606.
U.S. Pat. No. 5,149,862 describes sulfate-doped zirconium dioxide as heterogeneous acid catalyst for the polymerization of tetrahydrofuran.
Commercial polytetrahydrofuran and THF copolymer products have to have a specified mean molecular weight, usually in the range from 650 to 5000 dalton, and a narrow molecular weight distribution. At the same time, their color numbers must not exceed particular limiting values. Thus, the color number has to be below 40 APHA, preferably below 20 APHA.
A disadvantage of the known homogeneously or heterogeneously catalyzed processes for the single-stage synthesis of PTHF or THF copolymers is that they do not allow all the abovementioned properties of the PTHF and/or the THF copolymers necessary for commercial products to be realized simultaneously. Thus, numerous after-treatment processes for purification and reduction in the color number of the PTHF and/or the THF copolymers are known. In these, the products obtained from the polymerization, for example as described in EP-A 424 791, are after-treated with hydrogen in the presence of a hydrogenation catalyst.
It is an object of the present invention to find an economical, single-stage process which makes it possible to obtain both PTHF and THF copolymers having a narrow molecular weight distribution and a low color number in high space-time yield and with high selectivity.
We have found that this object is achieved by a process for the single-stage preparation of polytetrahydrofuran (PTHF) and/or tetrahydrofuran copolymers (THF copolymers) by polymerization of tetrahydrofuran over a heterogeneous acid catalyst in the presence of at least one telogen and/or comonomer selected from the group consisting of alpha,omega-diols, water, polytetrahydrofuran having a molecular weight of from 200 to 700 dalton and cyclic ethers, which comprises
a) separating off the catalyst and/or downstream products of the catalyst suspended and/or dissolved in the output from the polymerization,
b) fractionating the resulting catalyst-free output from the polymerization in at least one distillation step to give a distillation residue comprising the polymerization product and at least one tetrahydrofuran fraction and returning at least part of the tetrahydrofuran fraction to the polymerization and
c) separating low molecular weight polytetrahydrofuran or tetrahydrofuran copolymers having a mean molecular weight of from 200 to 700 dalton from the distillation residue from work-up step b) and isolating PTHF and/or THF copolymers having a mean molecular weight of from 650 to 5000 dalton.